Ultrasonic diagnostic apparatuses for obtaining a tomographic image of a subject using ultrasonic waves for medical diagnosis have been proposed. In such an ultrasonic diagnostic apparatus, when transmitting ultrasonic waves from an ultrasonic probe, so-called transmit focusing in which a transmit delay time is set to each ultrasonic wave transmitted from each element of the ultrasonic probe is performed and when obtaining receive signals, so-called receive focusing in which a receive time delay is set to each signal received by each element is performed in order to improve azimuth resolution.
When such transmit focusing and receive focusing are carried out, a representative sound velocity of the diagnostic target region is assumed and transmit delay times and receive delay times are set based on the assumed sound velocity.
But, the ambient sound velocity of a subject is not constant and differs from tissue to tissue, and if the assumed sound velocity differs from the ambient sound velocity, a problem of image quality degradation may arise.
One reason for image quality degradation is that the transmit delay times of transmit focusing or receive delay times of receive focusing differ from the transmit delay times for properly forming a transmit focal point on a target or receiving times of ultrasonic waves reflected from the target and received by the respective elements of the ultrasonic probe.
The term “ambient sound velocity” as used herein refers to a sound velocity determined based on the distance from a predetermined target to each element and the receiving time of each element when the ultrasonic wave is transmitted to the target.
Attempts have been made to prevent the image quality degradation by matching the assumed sound velocity with the ambient sound velocity.
For example, Japanese Unexamined Patent Publication No. 2007-007045 proposes the following. First, ultrasonic waves delayed so as to converge at a focal point T are transmitted from an opening having a predetermined width centered on the predetermined center of the opening at a normal sound velocity setting.
Then, reflection waves are received by all elements of the ultrasonic probe and receive focusing is performed on the signals with respect to focal points P1 and P2 at different sound velocity settings to generate a beam profile with respect to each sound velocity setting corresponding to each focal point.
Then, a beam profile having a smallest half width of the beam profiles of the respective focal points is selected, and the sound velocity setting corresponding to the selected beam profile is assumed to be the ambient sound velocity of the region of the subject.
In the method described in Japanese Unexamined Patent Publication No. 2007-007045, if the receive focal points P1, P2 are viewed as the attention points for obtaining ambient sound velocities, each of the attention points differs from the transmit focal point T in depth. In the case where the attention points are located adjacent to the focal point T, the ambient sound velocity may be obtained with satisfactory accuracy, but as the distance between them becomes large, the accuracy of the ambient sound velocity is degraded. In the case of speckle, in particular, the accuracy is degraded significantly due to interference from around the attention point and sometimes there may be a case that the ambient sound velocity cannot be obtained.
Further, in the method described in Japanese Unexamined Patent Publication No. 2007-007045, if the ambient sound velocity differs from the sound velocity setting, the transmit focal point differs from the transmit focal point T in depth as the transmit focal point T is at a depth where the ultrasonic waves converges at a normal setting sound velocity. In this case, the distance between the attention point and transmit focal point becomes large and the accuracy of ambient sound velocity is largely degraded so that sometimes there may be a case that the ambient sound velocity cannot be obtained.
Further, even if the attention points are located adjacent to the transmit focal point T, a correct ambient sound velocity may not be sometimes obtained due to interference. That is, the transmission wavefront becomes like that illustrated in FIG. 11 adjacent to the focal point, but a reflected wavefront identical to the transmission wavefront is falsely formed due to reflections from countless scattering points in each depth, resulting in an error in the ambient sound velocity obtained. More specifically, as illustrated in FIG. 11, the ambient sound velocity becomes fast in a region shallower than the transmit focal point and slow in a region deeper than the transmit focal point. FIG. 12 is a graph illustrating the ambient sound velocity with respect to the depth in the case where the error in the ambient sound velocity described above occurs.
In order to solve the problem, a method in which sound velocity setting of the transmit focus is changed, as well as the receive focus, with respect to the attention point is conceivable. But it requires a huge number of transmission times to change the sound velocity setting and transmits the ultrasonic waves each time the setting is changed. This takes too much time to obtain the ambient sound velocity, resulting in degradation in the processing or diagnostic efficiency.
The present invention has been developed in view of the circumstances described above and it is an object of the present invention to provide a focal point information determination method and apparatus and an ambient sound velocity obtaining method and apparatus capable of obtaining an ambient sound velocity successfully without causing degradation in the processing or diagnostic efficiency.